During the past several years, organic thin-film transistors (OTFT) based on π-electron materials have been extensively investigated for applications where current inorganic semiconductors cannot be employed. One, but not the only, arena for this new technology will likely include low-cost electronic devices such as solution processed/printed circuits for “smart” cards, radio frequency ID tags, flexible large-area displays, and eventually flexible active-matrix LCD/LED screens. One criteria is the availability of organic semiconductors having, besides obvious stability under operation, requisite TFT performance (carrier mobility μ>0.01 cm2/Vs; current on:off ratio Ion:Ioff>105) at sufficiently low operating source-drain/gate voltages (VSD/VG) and gate leakage currents to minimize power consumption. Many p-type and, to a lesser degree, n-type organic semiconductors exceed such metrics; however biases typically required to achieve such performance with conventional dielectric materials (silicon oxide, polymers, etc.) are unreasonably high for practical use (e.g., 50-100 V). Furthermore, relatively few studies have addressed the crucial issue of substantially reducing OTFT operating voltage, although the quest for thin “high-k” dielectrics is a major focus of current inorganic semiconductor research.
The source-drain current (ISD) in the TFT linear operating regime is expressed by Eq. 1, where W and L are the TFT channel width and length, respectively, VT is the threshold voltage, and Ci is the dielectric capacitance per unit area (Eq. 2 where k is the dielectric constant, ε0 is the permittivity of vacuum, and d is the dielectric layer thickness).
                              I          SD                =                              W            L                    ⁢          μ          ⁢                                          ⁢                      Ci            ⁡                          [                                                V                  G                                -                                  V                  T                                -                                                      V                    SD                                    2                                            ]                                ⁢                      V            SD                                              (        1        )                                Ci        =                              ɛ            0                    ⁢                      k            d                                              (        2        )            
For a given OTFT geometry and semiconductor material, similar current gains can be achieved at lower operating biases by increasing Ci. This is a useful relationship for OTFTs considering the relatively modest μ values (typically <1 cm2V−1s−1 vs. 103 cm2V−3S−1 for crystalline Si) exhibited by most organic semiconductors. Accordingly, recent approaches to increasing OTFT dielectric layer Ci have been to employ vapor-deposited inorganic materials having k higher than commonly used SiO2, such as Si3N4, BaSrxTi1-xO3, Ta2O5, and TiO2 or to use solution phase self-assembled monolayers (SAMs) of simple monofunctionalized hydrocarbon chains as OTFT dielectric layers. That is, the strategies applied to date can be summarized as either increasing k or reducing d while minimizing leakage currents. While some success has been realized, each strategy has limitations.